Optical disc drives employ a laser beam emitted from a semiconductor laser in a manner such that the laser beam is condensed by objective lenses into a small spot of beam (hereinafter called beam spot) focused on a disc surface for writing or reading information to/from the disc surface. In a magneto-optical disc for example, the disc surface is formed with a spiral guide channel (“groove”) for guiding the beam spot, and information is recorded in a ridge (“land”) which is the region sandwiched by the grooves. The land represents the information recording track, and one lap of the track is divided into a plurality of sectors (the unit of information recording).
Therefore, in information recording and reproducing to and from the tracks formed on the disc surface, beam spot access control is performed to move the beam spot onto a desired track, to read address information in each sector on the track, and to position the beam spot at a place (sector position) where desired information is to be recorded or replayed.
With the above principle in the optical disc drives, in order to move the beam spot radially of the disc thereby bringing the spot on the target track selected from a plurality of tracks (this operation is called “seek operation” below) as quickly as possible, the following arrangements are made. Specifically, the optical elements for generating the beam spot (the semiconductor laser, the objective lenses, a tracking signal detection system, a focus signal detection system, and so on) are mounted on an optical head, which is commonly made movable radially of the disc. Further, optical elements, such as the objective lens, which provide beam spot control in the directions of the optical axis are made finely movable radially of the disc, independently on the optical head. With these arrangements, the seek control is performed by doing two kinds of seek operations; a seek operation in which the entire optical head is moved radially of the disc over a long span to bring the beam spot close to the target track (rough seek operation), and another seek operation in which only the beam spot is moved radially of the disc minutely by the objective lens to bring the beam spot accurately on the target track (precise seek operation).
An optical head which is used for the seek control as the above includes an actuator for moving the entire optical head radially of the disc (hereinafter called carriage actuator) and an actuator for moving the objective lens slightly to move only the beam spot radially of the disc (hereinafter called lens actuator).
FIG. 15 shows a basic actuator configuration of an optical head which includes a carriage actuator and a lens actuator.
An optical head 10 includes a carriage 101, a carriage actuator 102 which moves the carriage 101 radially of the disc, an objective lens 104 supported by four springs 103 on the carriage 101, and a lens actuator 105 which moves the objective lens 104 radially of the disc independently. It should be noted that the figure does not show optical systems mounted on the carriage 101, e.g. a focus detection system, a tracking signal detection system, and so on.
The carriage actuator 102 includes a motor 106 which is a power source for the carriage 101 and is provided by e.g. a stepping motor, and a transmission member 107 which transforms the rotary power of the motor 106 into a linear motion power and transmits the power to the carriage 101. The transmission member 107 includes a shaft 107a which is connected with a rotor of the motor 106 and has a circumference formed with a male thread, and a first support 101a which extends out of a side surface of the carriage 101 (the upper side surface as in FIG. 15) and has an end formed with a female thread mated by the shaft 107a. In addition, the other side surface of the carriage 101 (the lower side surface as in FIG. 15) is provided with a second support 101b which extends out of the surface. The second support 101b has a through hole fitted by a guide rod 108 which runs parallel to the shaft 107a. It should be noted that the shaft 107a and the guide rod 108 are parallel to a radius of the disc.
Therefore, when the motor 106 is turned, the motor's rotating power is transformed to a linear motion power and transmitted to the first support 101a by the transmission member 107, causing the carriage 101 guided by the guide rod 108 to move radially of the disc, thereby moving the beam spot radially of the disc over a large distance (rough seek operation).
On the other hand, the lens actuator 105 includes a pair of magnets 105a, 105b provided on two side surfaces of a housing for the objective lens 104, and a pair of electric magnets 105c, 105d opposed respectively to the magnets 105a, 105b. The magnets 105a, 105b and the electric magnets 105c, 105d are placed in line, in parallel to the guide rod 108 (i.e. in parallel to a radius of the disc).
When no electricity is applied to coils of the electric magnets 105b, 105b, the objective lens 104 which is supported by the springs 103 is at a neutral point M (hereinafter the neutral point M is called reference position M). When electricity is applied to the coil of the electric magnet 105c or of the electric magnet 105d, the attracting force from the electric magnet 105c or the electric magnets 105d dislocates the object lens from the reference position M. The amount of dislocation is dependent on the amount of electricity applied to the relevant electric magnet 105c or electric magnet 105d. 
Therefore, by controlling the amount and the direction of electric power applied to the electric magnet 105c or the electric magnet 105d, the objective lens 104 is moved on the carriage 101, independently from the carriage 101 and radially of the disc, whereby the beam spot is moved radially of the disc by a minute distance (precise seek operation).
Now, there is a problem in this two-step seek control in which the carriage 101 is moved for a macro seeking and then the objective lens 104 is moved for a micro seeking. Specifically, due to the supporting structure that the objective lens 104 is supported by the carriage 101 via the springs 103, vibration occurs in the objective lens 104 when the macro-scale seek operation by the movement of carriage 101 is followed by the micro-scale seek operation by the movement of objective lens 104 as the carriage 101 accelerates or decelerates quickly, and it is impossible to start the seek operation by the objective lens 104 until the vibration ceases.
In an attempt to solve this problem, a number of seek operation methods have been proposed.
For example, JP-A-H09-223317 discloses a method in which a carriage travel distance is calculated on the basis of control signal sent to a carriage drive, a beam spot radial travel distance is calculated based on the number of crossings over disc tracks made by the beam spot, and the objective lens actuator is controlled so that the carriage travel distance and the beam spot travel distance are equal to each other.
According to this seek control method, it is stated that the carriage travel distance is the travel distance radially of the disc, so when the carriage travel distance is maintained equal to the beam spot travel distance radially of the disc in the seek operation, the carriage and the objective lens travel virtually at the same speed, without causing vibration in the objective lens, and so it is possible to improve efficiency in the seek control in which a rough seek operation by moving the carriage is used in combination with a precise seek operation by moving the objective lens.
JP-A-H08-147718 discloses a different method: The traveling speed of an optical head (the carriage) is detected at the time of seek operation, and the objective lens traveling speed is detected from a tracking error signal which is a control signal for automatically adjusting the beam spot hitting position on the track. Further, a relative speed of the objective lens to the optical head is calculated from the difference between the optical head traveling speed and the objective lens traveling speed, and the vibration of the objective lens is controlled on the basis of this relative speed.
In this seek control method, the carriage and the objective lens traveling speeds are used as parameters, and the seek control is made to zero the relative speed of the objective lenses to the carriage so there will not be vibration generated in the objective lens. This method and the method disclosed in JP-A 9-223317 Gazette are based on the same strategy that relative movement of the objective lens with respect to the carriage is controlled in order to reduce the vibration generated in the objective lens.
In the above, the beam-spot disc-radial traveling speed VA is obtained as follows: Tracking error signal (hereinafter called TES signal) has a sine-wave pattern as shown in FIG. 16(a) By comparing the TES signal to the zero-level signal (i.e. an average value of the positive-side peak value and the negative-side peak value as in FIG. 16(a)), a square-wave representation or square-wave tracking zero-cross signal (hereinafter called TZC signal) is made as shown in FIG. 16(b). Calculations are made to obtain a trailing edge time period or a rising edge time period TA of the TZC signal. The speed VA is obtained by dividing the track pitch XA (the pitch between the lands; see FIG. 17) by the time period TA.
There is a problem in this calculation method which uses the TZC signal in order to obtain the beam spot traveling speed VA. Specifically, when the beam spot has crossed a place where the groove and the land do not alternate each other, the TES signal will not make a sin wave form, and the beam-spot track-passing time TA calculated from the TZC signal is different from a correct value and therefore, the beam spot traveling speed VA is not accurate.
More specifically, the track on the magneto-optical disc is divided into a plurality of sectors, and each sector has a forefront portion which contains sector ID information including a track number, a sector number and so on. As shown in FIG. 17, the region in the land where the ID information is formed (pits) is pitted, i.e. formed with recesses. The height of the place where the ID information is formed is more or less the same as the height of two grooves which sandwich the land. For this reason, when the beam pitch crosses the region where the ID information is formed as indicated by Arrow Q, the waveforms of the TES signal and the TZC signal will be as shown in FIG. 18. Specifically, the number of track crossing is counted one time less. The beam-spot track-passing time TA then is longer than the correct value, which means that the calculated traveling speed VA is slower than the correct value.
When such a situation as the above takes place in the seek control where, for example, the relative speed of the objective lens with respect to the carriage has to be zero, it becomes impossible to control the relative speed accurately to zero during the seek operation, and it becomes unable to suppress the objective lens vibration stably or reliably during the seek control.